The present invention relates to optical picture scanning devices, particularly to devices which include a polygonal cylinder for line scanning.
Devices of this type are already known in the art and are disclosed, for example, in U.S. Pat. No. 3,977,760 and 4,002,830. However, it is also known that the optical system of such a device causes image distortions, such as astigmatism, curvature of the image plane and parallel shift of the central beam, which may well render the image blurred and out of focus. Efforts have therefore been made for eliminating these image distortions, for example, by inserting into the beam path a correcting lens computed in accordance with the optical data of the polygonal cylinder and the objective lens.
Inserting such a corrective lens can, however, only eliminate the curvature of the image plane and the parallel shift of the central beam while the astigmatism is rather increased by such a corrective lens. The astigmatism can only be corrected by corrective steps at the polygonal cylinder itself, as is well known to those skilled in the art.
Undertaking corrective steps at the cylinder itself would normally result in curved faces of the polygonal cylinder which so far are always plane parallel surfaces. Such curved faces, which would have a certain refracting lens effect, are extremely difficult to manufacture and would entail high costs and a high number of rejections.
It is therefore an object of the invention to provide simple means for correcting the astigmatism in optical picture scanning devices.